Referring to FIG. 1, a schematic representation is shown of a well being drilled for the production of hydrocarbons. Generally, a drill string 10 with a drill bit 12 is employed as is shown in borehole 14. Drilling mud is circulated downwardly through the drill string, as shown by arrow 16. The mud circulates downwardly through the string, passes out through the drill bit and upwardly through the annulus surrounding the drill string, eventually exiting therefrom, as shown by arrow 18.
Drilling mud is employed in the drilling operation for many reasons. For example, mud provides lubrication for the cutting surfaces and promotes the cutting action. In addition, it carries heat away from the cutting surfaces, reducing wear, and carries the cuttings away so as to constantly expose a fresh earth part of the formation. However, one of the purposes of a drilling system is to provide weight to the formation above the point of drilling. This is a safety feature. Should a gas pocket be encountered, it will naturally try to push upwardly to escape. The mud prevents this from occurring as violently and dramatically as would otherwise be the case by providing a weight on top of the exposed formation including gas. Different mud compositions vary greatly: however, a common mud weight is 18 pounds per gallon.
When gas, usually methane gas, seeps into the borehole from the formation during drilling, a bubble of such gas exists in the borehole under a great deal of pressure, for instance 8,000-10,000 pounds per square inch. The gas bubble, of course, displaces a small amount or volume of mud, as shown at bubbles 20 in FIG. 1. The mud is heavy and viscous, but a bubble entering the borehole near the bottom will rise and as it rises at bubble 22, then bubble 24, then bubble 26, and finally at bubble 28, the bubble becomes larger and larger because not as much pressure is applied to the bubble at the top of the borehole as at the bottom. Hence, it may be evident, that when there is gas in the mud, some of the volume previously occupied by relatively heavy mud is occupied by the gas and the weight of the column is much reduced compared with gas-free mud. If allowed to continue, this process will accelerate, possibly to the point of an uncontrolled blowout resulting in loss of the well and extensive damage. Also, the presence of increasing gas in the mud is an indication, a warning as it were, of approaching a gas formation to which the drilling is directed.
In FIG. 2, there is shown a conventional apparatus used for detecting gas and drilling mud, including mud reservoir 50 to which drilling mud 52 is introduced through inlet 54 after being circulated through wellbore 14 as illustrated in FIG. 1. Suspended or dissolved within the drilling mud may be found various amounts of numerous gases as well as other products, to which the drilling mud was exposed during subsurface circulation. Typically, as mentioned above, the primary gas constituent part is methane gas although other gases, such as butane and isobutane, are frequently present as well. The drilling mud is extracted from reservoir 50 through outlet 56 as needed, to be cleaned of contaminants and reused.
In order to extract a sample of the gas suspended within the mud, container 58 is partially immersed within the drilling mud. Container 58 is open ended and is positioned to allow drilling mud 52 to partially fill interior 60 of the container. Mechanical agitator 62 is mounted with container 58 and includes rotatable shaft 64 extending downwardly into drilling mud 52. Propeller 66 is mounted on the shaft and is rotated within the drilling mud by power unit 68. Rotation of propeller 66 mechanically agitates the drilling mud, tending to dislodge dissolved or suspended gases therefrom. The gases, being less dense than the drilling mud, emerge from the surface thereof and continue to rise as at 69 under influence of subsequently emerging gas.
The gas eventually rises out of chamber 58 and encounters open framework 70 mounted on the container. The framework supports funnel or collector 72 positioned over the center of container 58. The gas flows into funnel 72, through conduit 74 and gas sensor 76 under the influence of vacuum pump 78 mounted with the conduit. The gas is then allowed to vent through outlet 80. Gas sensor 76 is responsive to the presence of gas flowing therethrough and produces an electrical signal which is transmitted over electrical connection 82 and displayed on meter 84. Frequently, gas sensor 76 is located remotely from reservoir 50 to enable monitoring activity to be accomplished at a distance or at a central control location.
As can be seen, the gas is exposed to environmental conditions after leaving interior 60 of container 58 and until entering funnel 72. Consequentally, gas sensor 76 may not be able to measure the actual concentration of gas contained in the drilling mud. In recognition of this, gas sensor 76 is designed to measure fluctuations in the relative level of gas, not an absolute amount. That is, a sharp increase of gas detected in the sample is indicative of dangerous down hole conditions or possibly the presence of a hydrocarbon deposit, as previously discussed.
However, this conventional approach suffers from several inefficiencies and limitations. The gas sample may be diluted by breeze 86 prior to entering funnel 72, or by the dispersement of a portion of the gas to the atmosphere, as at 88. This prevents the gas sensor from reliably detecting variations in the amount of gas, since the measurement would not be more consistent than the force and direction of the breeze or other environmental conditions.
Additionally, a puncture anywhere along the length of conduit 74, even if of small dimensions, will be troublesome as the negative pressure existing therein will draw atmospheric gases into the sample stream to dilute the sample. Finally, dependence on a vacuum to gather and transport the gas sample is inefficient, in that methane and the other gases frequently encountered are heavier than air and resist mechanical efforts to force these heavier gases to flow upward until the influence of the vacuum pump takes over. Even then, these gases tend to "bunch", and produce irregular fluctuations in gas detector readings.
Therefore, it is a feature of the invention to provide an improved gas detector which measures the absolute quantity of the gas suspended or dissolved in a viscous liquid.
It is another feature of the invention to provide an improved gas detector which accurately senses variations in the level of gas suspended or dissolved in a viscous liquid.
It is yet another feature of the invention to provide an improved scheme to agitate and dislodge a gas dissolved or suspended in a viscous liquid by introducing pressurized air into the viscous liquid at a controlled rate.
It is still another feature of the invention to provide an improved scheme to utilize pressurized air to transport a gas sample from a sealed chamber to a remote gas sensor.
It is yet another feature of the invention to provide an improved method and apparatus for accurately measuring the absolute quantity of methane gas or the like dissolved or suspended in drilling mud circulated through a wellbore during drilling operations.
These and other objects, advantages and features of the invention will be apparent to those skilled in the art from consideration of the specification, including the attached drawing and appended claims.